Great question, and honestly, uneven ground is where most bench installations go wrong if you don't plan ahead. You want something that keeps the bench level, prevents wobbling, and won't sink or shift over time. Here are the footers I'd suggest, depending on how uneven or soft the ground is.
First, for mild unevenness or compacted soil, go with precast concrete pads or pour small concrete footers. Dig down to stable soil or gravel, level each pad independently with a spirit level, and set the bench legs on top. Use a layer of crushed stone under each pad for drainage. This works well on slopes up to about five degrees. You can shim between the leg and pad with stainless steel or plastic shims, but avoid wood since it rots.
Second, if the ground is really uneven, rocky, or you're working on a slope, helical piles or ground screws are the best choice. These screw into the ground with a drive tool, and you can adjust the exposed height by choosing different pile lengths or using a threaded top plate. They handle lateral forces well, so the bench won't tip. Plus, they create almost no excavation, which is great for tree roots or utility lines.
Third, for loose, sandy, or soft soil, a compacted gravel base under a wider footer plate is often enough. Dig a hole about 12 to 18 inches deep, fill with angular gravel in 4-inch lifts, and compact each lift with a hand tamper. Then set a precast concrete footer block on top and level it. This spreads the load and prevents sinking. If you're in a freeze-thaw area, make sure the gravel extends below the frost line or use a perforated drain pipe to keep water from pooling under the footer.
Fourth, for temporary or seasonal benches, use adjustable screw-type feet attached to the bench legs. These let you fine-tune each leg's height by rotating the foot up or down. But they only work if the ground is somewhat firm and the slope isn't too steep. Pair them with a wide rubber or polyurethane pad to reduce point pressure.
A few practical tips regardless of footer type: always set the bench so the seat is level, not the legs. If one side is higher, don't just bury the low side deeper because that creates a tripping hazard and water collection. Instead, build up the high side with a thicker footer or use a longer leg. And always anchor the bench to the footer with bolts or brackets, especially if the ground is sloped, so it doesn't get pushed out of position by wind, snow, or people sitting hard. Finally, check local codes — some cities require a minimum footer depth or specific anchoring in public spaces.